Tunable Anisotropic Wettability of Rice Leaf‐Like Wavy Surfaces

Abstract

Rice leaves can directionally shed water droplets along the longitudinal direction of the leaf. Inspired by the hierarchical structures of rice leaf surfaces, synthetic rice leaf‐like wavy surfaces are fabricated that display a tunable anisotropic wettability by using electrostatic layer‐by‐layer assembly on anisotropic microwrinkled substrates. The nanoscale roughness of the rice leaf‐like surfaces is controlled to yield tunable anisotropic wettability and hydrophobic properties that transitioned between the anisotropic/pinned, anisotropic/rollable, and isotropic/rollable water droplet behavior states. These remarkable changes result from discontinuities in the three‐phase (solid–liquid–gas) contact line due to the presence of air trapped beneath the liquid, which is controlled by the surface roughness of the hierarchical nanostructures. The mechanism underlying the directional water‐rolling properties of the rice leaf‐like surfaces provides insight into the development of a range of innovative applications that require control over directional flow.

Layer-by-layer Nanoarchitectonics: Invention, Innovation, and Evolution

“…Cho demonstrated tunable anisotropic wettability of rice leaflike wavy surfaces using electrostatic LbL assembly on anisotropic microwrinkled substrates. 342 Anisotropic wettability and hydrophobic properties could be controlled between the anisotropic pinned, anisotropic rollable, and isotropic rollable water droplet behavior states. LbL assembly at different pH values and for different numbers of polyelectrolyte and nanoparticle deposition cycles permitted control of surface roughness and porosity.…”

Linearly and Highly Pressure‐Sensitive Electronic Skin Based on a Bioinspired Hierarchical Structural Array

“…Hierarchical structures on the surfaces of organs, for example, can be critical for the survival of the organism: the maximized contact area and intermolecular interactions (van der Waals forces) resulting from the hierarchical structures on the feet of the gecko enable this animal to climb a vertical wall, [21] and the self-cleaning ability of the lotus leaf is attributed to the ability of the hydrophobic hierarchical structure on its surface to trap pockets of air. Dry adhesion, [22] wetting control, [23][24][25] and filtration membranes [26] have in fact been developed by mimicking hierarchical structures from nature. In the field of pressure-sensitive electronic skin, Ha et al [27] used a hierarchical structure composed of Pt-coated ZnO nanorods on a polydimethylsiloxane (PDMS) microdome array; the authors demonstrated a high-sensitivity (6.8 kPa −1 ) dual-mode pressure sensor based on piezo-resistive and piezo-electric transductions.…”

Fabrication of an Anisotropic Superhydrophobic Polymer Surface Using Compression Molding and Dip Coating

“…These trends are in good agreement with the findings of Lee et al and can be explained based on the surface roughness and surface free energy. A large contact angle hysteresis pins water droplets on the surface at a low surface roughness of a nanostructure, whereas a small anisotropic contact angle hysteresis generates directional movement of the water droplet, as observed on a rice leaf, and a water droplet on the surface with patterns tends to slide more easily along the direction parallel to the patterns due to the lower energy barrier for wetting [20]. In this research, the apparent contact angle on the flat surface with coating was 137 • , which is much larger than the apparent contact angle (98 • ) on the flat surface without coating due to the low surface free energy by silanization and increased nanoscale surface roughness resulting from the TiO 2 nanoparticle coating.…”

“…The lines or grooves have been fabricated by lithography, embossing, imprinting, laser machining, the formation of wrinkles via mechanical compressing, or other fabrication methods [13,[18][19][20][21][22][23][24][25][26][27][28]. For mass production of an anisotropic wetting surface, replication of patterns using a mold seems to be suitable, but many studies have used poly(methyl methacrylate) (PDMS), which has a relatively long curing time and limited reusability of the mold [13,[18][19][20][21], and molds with an uncontrollable surface roughness and side wall slope by laser machining and wrinkle formation, which does not show a clear pattern shape [7][8][9][10][11]13]. Additionally, the high surface energy ink pattern printing method can create various anisotropic superhydrophobic surfaces with low cost and large scale production especially 2D paper based applications [29].…”